Expression of the MRP and MDRI Multidrug Resistance Gene Products in 160 Untreated Human Carcinomas Studied by Immunohistochemical Methods in Formalin-Paraffin Sections

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Abstract
An immunohistochemical method was developed and applied to detect multidrug resistance related protein (MRP) in sections of formalin-fixed, paraffin-embedded human tissues. Monoclonal antibodies MRPm6, MRPrl, and QCRL-1 were used on sections of paraffin-embedded cell pellets of known MRP expression (HL60, HT1080, HeLa). None of the antibodies succeeded without pretreatment, but microwave epitope retrieval with 6M urea resulted in excellent specific staining with MRPm6. Moderate or weak staining was seen with MRPrl and QCRL-1. Various carcinomas were tested for MRP with MRPm6, and for MDRl P-glycoprotein (Pgp) expression with MAb JSB-1, by our previously described method (Am J Pathol 144:227-236, 1994) to investigate the possible coexpression of multidrug resistance markers in the same solid tumor. The incidence of positive immunoreactions/case with MRPm6 and JSB-1 respectively, in various human cancers was as follows: lung, 15 and two of 26; esophagus, eight and two of 15; head and neck, nine and one of 27; colorectal, 13 and 11 of 20; breast, 25 and 23 of 55; bladder, 0 and 0 of seven; ovarian, 0 and 0 of 10 cases. The overall incidence of MRP expression in these tumors was higher 70/160 (44%) than that of Pgp 41/160 (26%). Among Pgp-negative tumors 38% proved to be MRP positive. Coexpression of MRP and Pgp was found in 25/160 cases (16%), which is statistically significant (p=.0 17). A relatively higher incidence of strong MRP-positive staining was found in lung and esophageal cancers (4/26 and 5/15); otherwise staining was weak to moderate. No detectable MRP was found in stromal cells and in normal organs. Thus, MRPm6 by this method allows detection of MRP overexpression versus normal cells in paraffin sections of archived surgical specimens for investigation of the clinical significance of MRP.

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